The rapid accumulation of genome sequences and protein structures during the last decade has been paralleled by major advances in sequence database search methods. The powerful Position-Specific Iterating BLAST (PSI-BLAST) method developed at the NCBI forms the basis of our work on protein motif analysis. In addition, Hidden Markov Models (HMM), protein profile-against-profile comparison implemented in the HHSearch method, protein structure comparison methods, homology modeling of protein structure and genome context analysis were extensively and increasingly applied. Furthermore, custom libraries of protein domain profiles as well as computational pipelines for novel domain identification have been developed and applied. The research performed over the last year, has led to further progress in the study of the classification, evolution, and functions of several classes of proteins and domains. In particular, a detailed analysis of the proteins and domain involved in the CRISPR-Cas adaptive immune response in bacteria and archaea has been performed and a library of approximately 400 Cas domain profiles has been constructed. This resource was applied for comprehensive classification of the CRISPR-Cas loci in microbial genomes and discovery of new CRISPR-Cas variants. Evolution of CRISPR-cas loci, which encode adaptive immune systems in archaea and bacteria, involves rapid changes, in particular numerous rearrangements of the locus architecture and horizontal transfer of complete loci or individual modules. These dynamics complicate straightforward phylogenetic classification, but here we present an approach combining the analysis of signature protein families and features of the architecture of cas loci that unambiguously partitions most CRISPR-cas loci into distinct classes, types and subtypes. The new classification retains the overall structure of the previous version but is expanded to now encompass two classes, five types and 16 subtypes. The relative stability of the classification suggests that the most prevalent variants of CRISPR-Cas systems are already known. However, the existence of rare, currently unclassifiable variants implies that additional types and subtypes remain to be characterized. We further extended the Cas domain analysis towards discovery of novel CRISPR-Cas systems. Microbial CRISPR-Cas systems are divided into Class 1, with multisubunit effector complexes, and Class 2, with single protein effectors. Currently, only two Class 2 effectors, Cas9 and Cpf1, are known. We describe here three distinct Class 2 CRISPR-Cas systems. The effectors of two of the identified systems, C2c1 and C2c3, contain RuvC-like endonuclease domains distantly related to Cpf1. The third system, C2c2, contains an effector with two predicted HEPN RNase domains. Whereas production of mature CRISPR RNA (crRNA) by C2c1 depends on tracrRNA, C2c2 crRNA maturation is tracrRNA-independent. We found that C2c1 systems can mediate DNA interference in a 5'-PAM-dependent fashion analogous to Cpf1. However, unlike Cpf1, which is a single-RNA-guided nuclease, C2c1 depends on both crRNA and tracrRNA for DNA cleavage. Finally, comparative analysis indicates that Class 2 CRISPR-Cas systems evolved on multiple occasions through recombination of Class 1 adaptation modules with effector proteins acquired from distinct mobile elements. We further completed a project on identification and characterization of membrane proteins encoded in prokaryotic virus genomes. Viruses are the most abundant and genetically diverse biological entities on earth, yet the repertoire of viral proteins remains poorly explored. As the number of sequenced virus genomes grows into the thousands, and the number of viral proteins into the hundreds of thousands, we report a systematic computational analysis of the point of first-contact between viruses and their hosts, namely viral transmembrane (TM) proteins. The complement of -helical TM proteins in double-stranded DNA viruses infecting bacteria and archaea reveals large-scale trends that differ from those of their hosts. Viruses typically encode a substantially lower fraction of TM proteins than archaea or bacteria, with the notable exception of viruses with virions containing a lipid component such as a lipid envelope, internal lipid core, or inner membrane vesicle. Compared to bacteriophages, archaeal viruses are substantially enriched in membrane proteins. However, this feature is not always stable throughout the evolution of a viral lineage; for example, TM proteins are not part of the common heritage shared between Lipothrixviridae and Rudiviridae. In contrast to bacteria and archaea, viruses almost completely lack proteins with complicated membrane topologies composed of more than 4 TM segments, with the few detected exceptions being obvious cases of relatively recent horizontal transfer from the host. The dramatic differences between the membrane proteomes of cells and viruses stem from the fact that viruses do not depend on essential membranes for energy transformation, ion homeostasis, nutrient transport and signaling. Taken together, these studies not only expand the known repertoire of protein domains but also lead to the discovery of novel biologically important functional systems in diverse organisms some of which are expected to have practical implications, e.g. in genome engineering.